Working with the Unexpected at Asteroid Bennu

byPaul GilsteronMarch 21, 2019

We know by now to expect surprises when we do something for the first time with a spacecraft. The latest case in point is OSIRIS-REx, which has revealed multiple unexpected facets of the asteroid Bennu, near which it has been operating since December. Consider the surface of the asteroid, a key factor in how the mission goes forward since this is a sample return mission, and that involves finding a place relatively free of surface debris from which to take the sample.

The problem: This smallest body ever to be orbited by a spacecraft turns out to be strewn with boulders. The original sample collection plan — christened Touch-and-Go (TAG) — will have to be altered, for it was dependent on a sample site with a 25-meter radius free of hazards. The OSIRIS-REx team has been unable to identify any site that meets those requirements. A new type of candidate site will have to be found, demanding higher performance using an updated sampling approach called Bullseye TAG that will be tailored for smaller sample zones.

Nonetheless, the OSIRIS-REx team remains optimistic:

“Throughout OSIRIS-REx’s operations near Bennu, our spacecraft and operations team have demonstrated that we can achieve system performance that beats design requirements,” said Rich Burns, the project manager of OSIRIS-REx at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Bennu has issued us a challenge to deal with its rugged terrain, and we are confident that OSIRIS-REx is up to the task.”

Image: Bennu’s surface is rockier than expected, creating challenges for the team whose mission is to scoop up a sample of pristine material and return it to Earth in 2023. Credit: NASA/Goddard/UA.

The larger issue facing asteroid investigations is the question of computer modeling. The reason scientists have assumed that Bennu’s surface would be generally smooth is that observations from Earth of the object’s thermal inertia and radar measurements of its surface roughness have been integrated into computer models that made this prediction. We now learn that the interpretation of these models was not correct. Indeed, data from Bennu should help us refine such models to better predict what we’ll find on the rocky surfaces of small asteroids.

A suite of papers covering the Bennu findings has appeared in Nature following presentations at the recent 50th Lunar and Planetary Conference in Houston (citations below). On the matter of Bennu’s boulder strewn surface, a team from SwRI presents results showing that the surface geology of the asteroid is between 100 million and 1 billion years old. That too can be considered a surprise, to judge from the remarks of SwRI’s Kevin Walsh:

“We expected small, kilometer-sized NEAs to have young, frequently refreshed surfaces,” said SwRI’s Dr. Kevin Walsh, a mission co-investigator and lead author of a paper outlining the discovery published March 19 in Nature Geoscience. “However, numerous large impact craters as well as very large, fractured boulders scattered across Bennu’s surface look ancient. We also see signs of some resurfacing taking place, indicating that the NEA retains very old features on its surface while still having some dynamic processes at play.”

Some of Bennu’s boulders are larger than 45 meters (150 feet) in size, much larger than earlier observations had predicted, and according to Walsh, they are simply too big to be the result of cratering. Instead, the scientist believes they date back to the formation of the asteroid.

But OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer — I have to untangle the acronym once in each post on the mission) continues to seize our attention long before the sample return with the news that particle plumes are erupting from its surface. This one caught everyone by surprise as well when, on January 6, the science team discovered the plumes while the spacecraft was about 1.6 kilometers away, with further detections in the ensuing months. Some of these particles were ejected from Bennu entirely, while others returned to the asteroid. None are thought to pose a danger to the spacecraft.

“The discovery of plumes is one of the biggest surprises of my scientific career,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. “And the rugged terrain went against all of our predictions. Bennu is already surprising us, and our exciting journey there is just getting started.”

Image: This view of asteroid Bennu ejecting particles from its surface on January 19 was created by combining two images taken on board NASA’s OSIRIS-REx spacecraft. Other image processing techniques were also applied, such as cropping and adjusting the brightness and contrast of each image. Credit: NASA/Goddard/University of Arizona/Lockheed Martin.

We’ve already talked about the change in Bennu’s spin rate as an apparent result of the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect (see Asteroid Bennu: Changes in Rotation Rate), a fascinating find in itself, but it’s also encouraging in terms of understanding asteroid composition to learn that the MapCam color imager and the OSIRIS-REx Thermal Emission Spectrometer (OTES) have detected magnetite on Bennu’s surface, which points to rock and liquid water interactions on the asteroid’s much larger parent body.

Learning about the sources of organic molecules and water on Earth may be enhanced by our analysis of such asteroids, and we’re also beginning to learn what resources will be available in near-Earth space. Bennu truly offers us a window into the early days of the Solar System.

If Bennu is an agglomerative composite body of stuff accreted from various sources in different eras over the past few billion years, do we have ways of sifting the samples and separating them into coherent batches?

What is Bennu’s average surface gravity? Also, how much centrifugal force does an object feel at its equator? Since objects are observed being flung off its surface I surmise that at its present rate of spin its net gravity must be completely cancelled out in places.

There isn’t much difference. I calculated that up, using old Wikipedia data and assuming a sphere (with a radius of 500 meters). The escape velocity of Bennu came out to about 0.3 miles/hour while the rotational (tangential) speed at the equator was about 0.46 miles/hour, which I didn’t believe. I know the formula I used for escape velocity is right, as it gives the correct value for the Earth……

Thanks for answering Martin. Your figures make sense though as a first approximation at least. Loose material observed leaving Bennu’s surface proves that at least in places its weak gravity is exceeded by centripetal “force”. YORP spin up has reached the point where it is most definitely reshaping this body.

Thanks, my radius (500 meters) was more like the diameter! Using 262.5 meters for the radius (I misread to Wikipedia page) I get your figures (to one significant figure). Converting 2-4 cm/sec to “American” units it’s .04 to .09 miles/hour.

@Laura, I simply imagine that a large number of small pebble like objects of eventually coalesced into the asteroid that you now see before you. The more interesting question is-how is this current body ejecting small pebble like objects and particles seemingly spontaneously.

Laura’s question is excellent however, as it goes to the heart of the biggest questions in planetary formation theory. With the force of gravity being so minuscule between very small objects, how can they coalesce into something massive enough to (1) begin gravitational accretion before (2) being totally disrupted by an impact?

Are there not other forces than gravity to make particles stick together?

In Bennu’s case, what I find interesting is this rubble landscape that stands in contrast to other visited asteroids. Has it always been thus, or have the smaller grains been removed from the surface, leaving the larger rocks? In our gravity field, jostling of mixed materials leaves the smallest at the bottom and the largest at the top. Is that even possible on such a small body with a weak gravity field?

Other forces besides gravity? The two nuclear forces can be ruled out due to extremely short range. That leaves electromagnetic, which definitely plays a role in clumping atoms and molecules together. Also it must help pellets grow in a manner like hailstones do as volatiles freeze onto their surfaces. Finding magnetite also proves electromagnetism’s involvement.

However, all those rocks and boulders sure don’t look look like their made of ices…

There are van der Waal’s forces that work on silica and metals, as well as volatiles. Carbonaceous material could “glue” particles together. Impacts might cause local melting and fusion of rocks. And so on. My point is that there is more to consider than just the simple forces of acceleration forces (g and centripetal) on inelastic, point-like particles.

Ah. I was thinking more fundamentally of course. (But please don’t take that to mean that I’m a Fundamentalist! Young Earth Creationists have spoiled the connotation of what it means to be a Creationist. My thinking is in line with men of science like Isaac Newton.)

@Bruce D. Mayfield, with regards to planetary formation theory I would answer in the following manner. Gravitational accretion would overpower impact disruption simply because the fact that accretion occurs over EXTREMELY long time periods, which is more than enough to overcome the miniscule force of gravity. Additionally, objects that begin to coalesce into any great sizeability undoubtedly have far more room between themselves than is generally acknowledged. Most of outer space is an fact, just nothing but empty space. Hence collisions and disruptions are far exceeded by weak gravitational accretions. Ultimately gravitational accretion wins out in the long run compared to the comparatively rare impact between objects.
Before space probes were sent through the asteroid belt there was a tremendous fear that they would be destroyed by impacts; in fact it turned out that the asteroid belt was pretty empty of everything and experience has shown that most of outer space has no sizable impactors to worry about.

YORP spin up may indeed be the answer as to why stuff may be cast off. In fact of I would say that all possibilities that are physically realizable could explain so many of the properties of not only this asteroid, but many others. My only difficulty with YORP as a potential mechanism for the rotational increase is the fact that so far we have not seeing this happening in other celestial bodies. We are observing asteroidal bodies now more than ever, and we know the light curves through constant monitoring and this appears to be the first (and so far only) one that observes this behavior (of rotational speed up). So why is this object only the one to exhibit this behavior. Kind of strange.

And I think it’s kind of important here that I explained my type of reasoning which pretty much goes across board in any problems that I encounter. Essentially it all boils down to visualization in the mind’s eye of how you weigh out which various physical processes will occur and what weight to give them.

Now I’m not claiming infallibility or am I saying that such a visualization will inevitably lead you to the right answer. Rather, the mind’s eye permits one to give an assessment within a ballpark region as to what process or processes will tend to be dominant over other possibilities.
Thus, I’m not dismissing anything that anyone else is saying, I’m simply weighing in my mind of what I considered to be the most likely outcome – YORP as a potential mechanism for the rotational increase could very well be the explanation, however, my intuition suggests that most bodies tend to be in a relatively uniform state and I would guess that more often than not that particular mechanism would not be a first choice candidate for increased rotation. I hope that my explaining my reasoning (which applies in so many other areas) will give you some insight as to how I think. Again, I’m not saying that other explanations, such as YORP as a potential mechanism for the rotational increase are not valid just that I would not put money down on it as my first choice.

I have to wonder about the conclusion that the surface is old, and whether those are _impact_ craters along the equator (Ryugu shows similar features along its equator.) I suspect they are “extraction” craters, where previously YORP spun-up the asteroid until it started loosing very large (45 m) boulders, which carried away angular momentum and changed the surface enough to stop or reverse the YORP spin-up for a while (YORP is very sensitive to surface features, both topographic and albedo). Extraction craters would be created looking eroded compared to impact craters, which means the surface would look considerably older than it actually may be.

The cover story for the Jan.-Feb. 2019 Analog SF magazine is Tim Jolly’s Ring Wave. A 20-kilometer asteroid strikes Earth, converting its entire surface to magma. There is one slim hope. The asteroid’s impact creates a ring-shaped wave, which shoots up so high and so fast that it could launch escape pods into orbit. So escape pods of various sizes are launched into space via the ring wave.
The story itself is just one woman’s fight for survival and against pirates.

The story states that space faring nations intend to send tens of thousands of colonists to Mars, but that that wouldn’t be enough. I don’t know how many would be “enough”?

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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